1. Field of the Invention
The present invention relates to a radio transceiver for transmitting and receiving electromagnetic waves through a rod-like antenna such as a whip antenna, etc., and, particularly, to a radio transceiver capable of maintaining the antenna gain of such a rod antenna in an entire signal frequency band for which it is used.
2. Description of the Related Art
In a conventional portable radio transceiver such as a portable telephone set, a rod-like antenna such as a whip antenna having an antenna element a half wavelength long has been used mainly. Such a rod antenna has the advantage that it provides substantially the same radiation pattern and antenna gain as those of a dipole antenna, with minimal variations, even when the transceiver is worn on the head of the user. Further, such a rod antenna improves the portability of the transceiver since it can be easily retracted within the transceiver set by reducing the length of the antenna element. Radiation impedance of such a rod antenna is as high as several hundreds ohms or more.
The radio transceiver further comprises a duplexer connected to a feed point of the rod antenna for separating a transmitting signal to be supplied from a transmitting section of the transceiver to the rod antenna from a receiving signal to be supplied from the rod antenna to a receiving section of the transceiver. However, since the input/output impedance of the duplexer is usually designed to be of the order of 50 ohms, it is necessary to provide an impedance matching circuit between the duplexer and the rod antenna for preventing reduction of antenna gain in an operating signal frequency range.
An example of such an impedance matching circuit is disclosed in Japan Kokai (P) Sho 63-176003 (publication date: Jul. 20, 1988). The disclosed impedance matching circuit has a construction of low-pass filter including an inductor connected in series with a transmission line and a capacitor connected in parallel to the same transmission line and functions to optimize impedance matching between a rod antenna and a transceiver at a frequency which is substantially an intermediate frequency between a transmitting signal frequency (referred to as "transmitting frequency") and a receiving signal frequency (referred to as "receiving frequency").
A frequency band in which the impedance matching circuit exhibits optimum impedance matching between the rod antenna and the duplexer is only several percent (%) of an intermediate portion between the transmitting frequency and the receiving frequency, where the impedance matching condition is sufficient to obtain a VSWR of not more than 2.0 (return-loss of 9.6 dB). Therefore, in such a radio transceiver, when the operating signal frequency band used is very wide or the transmitting frequency is much different from the receiving frequency, it is impossible to obtain good impedance matching between the antenna and the duplexer in a desired signal frequency band even if such an impedance matching circuit is used, and thus reduction of effective antenna gain over the operating signal frequency band except a portion thereof.